Look back at BBC programme (Building Cars live BBC2 19.30hrs Tuesday this week) on Cowley works Mini production. Good portion on spot weld testing using an U/S probe nd also tearing them apart in the classical way.

In a previous life I used to do a lot of NDT (no destructive testing) of welded joints, so the methods available are as follows: dye pen, mag particle, ultrasonic and x-ray.

What you can do yourself is limited to dye penetration testing and magnetic particle testing. The Dye Pen method is to clean the test area with a special cleaner, spray the test area with a dye, usually red, and let it dry, then clean it off with a cleaner 'soaked' (lightly coated) rag, then spray with a very light coat of a usually white topping. The crack will show up as a red line. Magnetic particle is much the same, clean, spray with a white coat, let dry, spray with a fluid containing iron filings and then apply a magnet. The filings line up along the crack. Although both methods are a bit on the DIY side, they do need the right instruction and application to enable the method to be done properly, so some training is required. Like most things the tests are useless if you don't know what you are doing.  A finer more sensitive method for both is a UV version of both – and for both these training is definitely required!

The problems with both these methods, good as they are, is that it only shows up surface defects on the tested side, not any defects within the weld. To do that you have to examine the weld using either ultrasonic testing or radiography (X-Ray), or both (boiler tubes on coal-fired power stations would employ 100% ultrasonic PLUS 10% X-ray). For both you need very well trained and competent personnel to carry out the test and interpert the results, an x-ray is not as easy to view and determine what it shows as one might think. X-ray testing is not foolproof either, the angle at which the image was taken can mean a weld with a crack within it can seem to show a perfectly good weld on the x-ray print. An example of this at the power station was one weld where the ultrasonic guy said there was a crack in the root of the weld but the x-ray was OK – I know as I also viewed it. The ultrasonic guy was adamant the crack was there, the weld was retested with a shot from a different angle and there was the crack clear as clear can be.

With ultrasonic testing, one basically used a probe which sent out an ultrasonic sound wave out and through the metal and an electronic screen which displayed what waves came back. The probes are produced to send the ultrasonic waves out at a predetermined angles from 0 deg. (straight down) to close to 90 deg. The equipment is calibrated before use by using known targets on the probe in use and so setting the screen to display the results to a certain magnitude. A 0 deg probe finds the thickness of the plate or tube being tested and also any laminations within the plate, the other probes send waves out to search the weld. If all is good the waves just disappear down the metal but any defects within the weld send a signal back (like radar) which is then plotted on a drawing, noting where the probe was when the signal was received, so a bit of maths was involved, to show what and where the defect is and likely to be, like lack of root penetration, root crack, slag inclusion and so on and so forth plus its size and position.

There are times when, having made an ultrasonic examination and you have put yourself on the line by saying for example that there is a crack in the boiler furnace tube from here to here and to within 3mm of the surface and therefore the boiler needs an expensive repair before being returned to service, you are rewarded by the boiler repair guys tell you after that the crack was exactly where you said it was, from there to there and to within ⅛" of the surface – that would do for me! There would also sadness when an ultrasonic test of a boilers riveted seams shows a massive amount of cracks at just about every rivet hole meaning the boiler was a scrapper. Especially when this boiler was at a wood 'factory' or yard and all the scrap wood and sawdust was burnt in this boiler to produce the steam the yard required for its processes, a very waste-not want-not system, and a sad day telling the manager his boiler was a scrapper, he'd have to get a new one.

Chris

Edited By ChrisH on 23/10/2015 14:48:43

Edited By ChrisH on 23/10/2015 14:50:58